1. Field of the Invention
The present invention relates to a data transmission method in radio multicast communication. Particularly, the present invention relates to a radio multicast communication in which when an error is detected in a multicast-transmitted packet, a negative acknowledgment (hereinafter referred to as NAK) is returned as a response to a base station to thereby perform a request for resending.
2. Related Background Art
When performing multicast communication in a radio communication system, there is an advantage that all terminal stations able to communicate with a base station can transmit/receive information and the information can be transmitted to all the terminal stations at one transmission. However, on the other hand, when performing a request for resending because of an error generated in a transmission line, if a plurality of terminal stations simultaneously perform requests for resending, there is a problem that the requests collide against one another on a radio circuit and resending request (NAK) information is not correctly transmitted.
The problem about signal collision caused when the plurality of terminal stations utilize the same circuit is well known as a multi-access problem, and various solution methods have been proposed.
For example, a transmission right control system of issuing signal transmission rights for transmission acknowledgment (Japanese Patent Application Laid-Open No. 46161/1999), a system of transmitting a NAK signal provided with a packet number not normally received by random access when an error is generated in a reception signal (Japanese Patent Application Laid-Open No. 210031/1998), and a system of transmitting a burst signal as the NAK signal to a time position corresponding to the packet number when the error of the reception signal is detected (Japanese Patent Application Laid-Open No. 53089/1993) are known.
In the first transmission right control system, transmission/reception of information for adjusting a returning timing of a resending request signal is necessary, and there is a problem that control is complicated. Moreover, in a mobile communication system in which the terminal station moves, since the terminal station as a multicast communication target changes, the control is further complicated.
In the second random access system, the resending request signal of the multicast communication is frequently generated in a plurality of terminal stations at the same time, a probability of occurrence of collision of the NAK signals is high and an efficiency is deteriorated. To reduce the collision, a back off time needs to be taken before the transmission of the NAK signal. However, when the number of multicast address terminal stations increases, the back off time needs to be increased, and efficiency deterioration attributed to the back off time cannot be ignored.
In the third burst signal system, similarly as the random access system, the probability of occurrence of collision is high. However, to detect a signal energy in the time position, even when the NAK signals from a plurality of terminal stations collide with one another, some signal energy is detected, and it can therefore be recognized that the corresponding packet is erroneously received by at least one terminal station. In this system, however, a signal energy detection precision raises a problem. For example, when two signals subjected to PSK modulation are received by a multipath with a phase deviating by 180 degrees, the signal energy becomes zero, and the base station as the multicast transmission station cannot detect that an error is generated in a reception station for receiving the packet.
Moreover, in the present system, since an erroneous packet is specified by the time position for transmitting the burst signal, with a detection miss (although the burst signal is received, it is judged that there is no burst signal) resending of the erroneous packet is not performed. In order to reduce the detection miss, when a threshold for detection is lowered, erroneous detection (although no burst signal is received, it is erroneously judged that there is a burst signal) is easily caused by influence of disturbances such as an undesirable noise, and unnecessary resending is performed.
That is, various solution methods of multiple access have been proposed, but there are problems such as a complicated control and an insufficient effect.
Incidentally, because of completion of IEEE 802.11 radio LAN standards in 1997, and advancement in price reduction of radio LAN, a large number of radio LAN products have been placed on the market.
At present, aiming at a higher speed of the radio LAN, in IEEE 802.11 committee, specifications of the radio LAN using a radio frequency of 5 GHz band are studied, and it is determined that an orthogonal frequency division multiplexing (OFDM) system strong against multipath interference is used as a transmission system.
On the other hand, in a current IEEE802.11 resending control method, when performing unicast transmission for transmitting information to one specific terminal station, if the transmitted packet is correctly received, the terminal station returns an acknowledgment signal (hereinafter referred to as the ACK signal) after a time interval called a short interframe space (SIFS).
However, for the multicast communication including the multicast transmission, no acknowledgment is made in the specifications. Specifically, since the resending control in a radio link is not applied, reliability of information transmission is low in the multicast transmission, and further there is a problem that the data transmission efficiency is lowered by the resending control of an upper layer.